The planet Mars we know today is a cold, dry, desert world, but
suppose the martian climate is changing even now, year to year and
decade to decade?

New observations by NASA's Mars Global Surveyor spacecraft are
expanding our understanding of the martian climate and may indicate the
climate is changing significantly even today. This suggests even larger
climate changes have occurred during the planet's recent history and
may again in its future. The observations were made during a full martian
year, 687 Earth days.

If this is so, Mars might someday become warmer and wetter, as
some scientists suggest it was during its early history. Papers detailing
these observations are published in the Dec. 7, 2001, issue of
Science magazine.

"If the environment of Mars has really changed by as much and
over as short a time-scale as our observation implies, there should be
attributes of Mars reflecting these changes that may be measurable by
landers," said Dr. Michael Malin, principal investigator for Global
Surveyor's camera system at Malin Space Science Systems,
San Diego. "If Mars had a higher atmospheric pressure in the
not-too-distant past, it is more likely that water was present as a
liquid near the surface."

Liquid water is required to support known forms of life, and the
presence of liquid water on Mars would make it more likely life may
once have existed there.

"Detecting evidence of climate change and variability on Mars
using Mars Global Surveyor data is an important aspect of telling us
where to go on the surface this decade," said Dr. James Garvin,
lead scientist for Mars exploration, NASA Headquarters, Washington,
D.C. "Clearly, the polar regions are a good place where we
would like to look for hydrothermal vents to see if they exist on Mars."

Images from Global Surveyor's camera system show that pits -- often
referred to as the "Swiss cheese" terrain -- at the southern
polar ice cap of Mars have dramatically increased in diameter, indicating
the material has evaporated rapidly compared to last year.

"The amount of change is much larger than any previous change
we've seen on Mars, and it is much larger than can be explained by the
evaporation of water ice. We have calculated the only material that could
have changed this much is carbon dioxide ice, what we know as dry
ice," said Malin. "This means the Mars environment we see
today may not be what it was a few hundred years ago, and may not be
what will exist a few hundred years in the future."

A separate observation is providing more detail about the behavior of
carbon dioxide in the martian atmosphere. Carbon dioxide is a
"greenhouse gas" believed to warm climates when its
atmospheric concentration increases. The spacecraft's laser altimeter and
radio tracking system have made precise measurements of the amount
and density of carbon dioxide snow in both polar regions. This information
gives scientists the first global measurement of the seasonal exchange of
carbon dioxide between the atmosphere and surface.

Due to the tilt of the planet, Mars has seasons just like Earth.
Scientists have long known the most important seasonal change on Mars
is the autumn and winter "freezing out" of carbon dioxide
from the atmosphere in the form of dry-ice frost and snow. The evaporation
of the surface frost in spring and summer returns carbon dioxide to the
atmosphere. Over the course of a martian year, as much as a quarter of
the atmosphere freezes out, but until now scientists didn't know precisely
where and how much dry-ice frost and snow would pile up on the surface.

"We have measured how deep the dry-ice snow got on Mars
over the course of a year. We have also measured the corresponding tiny
change in the gravity field due to carbon dioxide being transported from
one pole to the other with the seasons," said Dr. Maria Zuber,
deputy principal investigator of the laser altimeter, at the Massachusetts
Institute of Technology, Cambridge, and NASA's Goddard Space Flight
Center, Greenbelt, Md.

"Snow on Mars is denser than snow on Earth and is really
more like ice than snow. Understanding the present carbon dioxide cycle
is an essential step towards understanding past martian climates,"
Zuber said.

JPL manages the Mars Global Surveyor mission for NASA's Office of
Space Science, Washington, D.C. JPL is a division of the California
Institute of Technology, Pasadena.